1. Field of the Invention
The present invention relates to a vertical semiconductor device, particular to a vertical insulated gate bipolar transistor (hereinafter, called as an “IGBT”).
2. Description of the Related Art
In an IGBT as a vertical gate type semiconductor device with high voltage resistance, as of now, since the gate circuit can be easily fabricated because the gate is voltage-controlled, the IGBT can be widely available a power electronics field typified by an inverter and a switching power source. The IGBT is a power device having high speed switching characteristics of a MOSFET and high output power characteristics of a bipolar transistor.
In general, the IGBT is configured to be separated into the effective unit (chip center) and the peripheral unit (chip terminal). If the IGBT chip center has only the effective unit, the dielectric breakdown and/or the low voltage avalanche breakdown of the IGBT chip may be caused due to the increase of the surface field effect at the periphery thereof and the increase of the internal electric field thereof. In this point of view, as described above, the IGBT is separated into the effective unit functioning as the inherent IGBT (transistor) and the electric field-relaxing unit (chip terminal) with the electric field relaxing structure to relax the surface electric field as described above, thereby enhancing the voltage resistance thereof.
For example, such an attempt is made as forming, on the surface of the n-type base layer constituting the IGBT, a resurf layer with P-type impurities injected therein and dispersing the surface electric charges to develop the voltage resistance of the IGBT (Reference 1). Moreover, such an attempt is made as forming, on the surface of the n-type base layer constituting the IGBT, a plurality of guard-rings with p-type impurities therein and dispersing the surface electric charge to develop the voltage resistance of the IGBT (Reference 2).
In the IGBT with the developed voltage resistance as described above, holes injected from the collector are spread over the IGBT. In the case that the IGBT is off-state, therefore, the depletion layer formed in the n-type base layer is contacted with the n-type buffer layer located below the n-type base layer at the effective unit of the IGBT so that the voltage and current can not be stable and may be oscillated after the off-state of the IGBT. As a result, the IGBT may cause some electromagnetic noises due to the oscillation of the voltage and current and thus, may cause noise sources for other elements and other instruments connected therewith. Accordingly, the entire operation of the device including the IGBT under good condition is disturbed. What's worse, the IGBT may be broken.
In order to reduce the oscillation of the voltage and current, it is considered to increase the thickness of the n-type base layer, but in this case, the electric power loss may be increased due to the increase of the thickness of n-type base layer.
Moreover, if the intensity of the electric field depending on the carrier density stored in the IGBT becomes larger than a prescribed electric field, the low voltage avalanche may occur so as not to realize the desired voltage resistance.
[Reference 1] JP-A 8-288524 (KOKAI)
[Reference 2] JP-A 2001-217420 (KOKAI)